Band-gap voltage regulators are typically used to provide substantially constant reference voltages in environments subject to temperature fluctuation. Generally, band-gap circuits develop a voltage proportional to the difference between base-to-emitter voltages, .DELTA.V.sub.BE, of two transistors to compensate for the temperature variation in the transistor base-emitter voltage to develop a temperature compensated output voltage. Because .DELTA.V.sub.BE is small (e.g., less than 100 mV), it is amplified to compensate the temperature variation in V.sub.BE. A disadvantage of amplifying .DELTA.V.sub.BE is that circuit noise is also amplified.
In U.S. Pat. No. Re. 35,951, issued to Ganesan et al., in an attempt to reduce noise, transistors are stacked to reduce the amount of amplification needed. Stacking transistors reduces the amplification needed and also reduces noise because, the .DELTA.V.sub.BE 's add directly and the noise from each transistor adds on a power basis. Because power is proportional to voltage squared, the ratio of the output voltage (after amplification) to noise increases (improves) by the square root of the number of stacked transistors.
Greater .DELTA.V.sub.BE values have been produced by stacking like transistor types. For example, stacking NPN transistors, stacking PNP transistors, and amplifying the difference in the cumulative .DELTA.V.sub.BE 's of each stack. This is illustrated in FIG. 1. In FIG. 1, NPN transistors are stacked together and PNP transistors are stacked together. For example, NPN transistors Q29, Q31, Q33, and Q35 are stacked together and PNP transistors Q21, Q23, Q25, and Q27 are stacked together.
A problem with this approach is that the minimum supply voltage needed to power a stack increases as the number of transistors in the stack increases. As previously stated, increasing the number of transistors in a stack decreases noise, but increasing the number of transistors in a stack also increases the minimum supply voltage required. Thus, a need exists for a low noise band-gap reference voltage that operates with a low supply voltage.